One large concern for many portable devices is battery power. With a limited power supply, these portable devices often try to limit the amount of power that is used at any given time. One way to accomplish this is to turn off all or part of the device's functions or send them into a sleep mode when they aren't needed. Then, when there is a call for that functionality, the device can waken the necessary circuitry and perform the required action. But when there is no need for that functionality, the circuits can remain off or in the sleep mode, and the level of power consumption for the device will be less than during normal operation.
This tactic can work well for operations that are largely under the control of the user. A display screen on a remote device might be powered down if no action is taken for a set time. Then, when the user once more needs to view something on the display screen, he can reactivate it. Nothing is lost by powering down the screen since nothing critical will happen while the device is powered down. If the at any time user needs to view something, he can simply reactivate the screen. And if the user has to wait a moment for the screen to come back on, nothing critical will be lost.
But some operations are less predictable and more critical, such as wireless communication. A wireless device may not necessarily know when it will receive a signal, and may be required to reply to such signals within a set period of time. As a result the device must remain alert at all times for incoming signals. For example, a cell phone could receive a call at any time; you can't predict when a homeowner might activate a garage door opener; and a wireless router may not be able to predict when it will have to pass data. As a result, a wireless receiver in such a device may have to remain in at least a listen mode for an extended period of time, even if the chance of receiving data is slim.
The level of power consumption for a receiver circuit can vary tremendously depending upon the complexity of the receiver circuit, however. A relatively simple receiver circuit that detects only non-coherent signals is comparatively cheap and low in power consumption. But non-coherent signals are not the most efficient for sending data, and so the use of non-coherent signals can significantly limit data transmission rates. A more complex receiver circuit that detects coherent signals is more expensive and higher in power consumption, but provides for a more efficient signal processing, allowing greater transmission speeds.
It is therefore desirable to provide a way to use the advantages of both coherent and non-coherent signal processing within a single device.